The present invention generally relates to trunk line identification systems, and more particularly to a trunk line identification system which is applied to a communication from a first station to a second station via a third station.
FIG. 1 generally shows an example of a conventional communication system. A station "A" is coupled to a station "B" via a transmission line 3, and the station "B" is coupled to a station "C" via a transmission line 4 Each of the stations "A" "B" and "C" have a switching system 1 and a transmitting apparatus 2. For example, each transmitting apparatus 2 compresses a pulse code modulation (PCM) signal of 64 kbps into 32 kbps by an adaptive differential pulse code modulation (ADPCM) coding scheme and transmits the compressed signal on the transmission line 3 or 4.
Among the signals transmitted from the station "A" to the station "B" there are signals which are to be dropped to a subscriber 5 within the station "B" and signals which are to be transmitted to the station "C" via the station "B".
According to the conventional communication system, the transmitting apparatus 2 multiplexes signals amounting to a plurality of channels (for example, 24 channels in the case of PCM 24 system) and simply transmits the multiplexed signal on the line. Hence, there is no function of judging whether the signal received from the station "A" via the transmission line 3 is to be dropped at the station "B" or relayed to the station "C" via the transmission line 4. For this reason, the transmitting apparatus 2 of the station "B" must once decode the compressed signal back into the original PCM signal, and the switching system 1 of the station "B" judges whether the PCM signal is to be dropped at the station "B" or relayed to the station "C" via the transmission line 4. If the PCM signal is to be dropped at the station "B", the PCM signal is dropped to the subscriber 5. But if the PCM signal is to be relayed to the station "C" the PCM signal is first returned to the transmitting apparatus 2 of the station "B" so as to be compressed again, and the compressed signal is then transmitted to the station "C" via the transmission line 4. As a result, the signal from the station "A" is relayed by the station "B" and transmitted to the station "C".
As described above, the switching system 1 of the relay station "B" must also receive the signal which is unrelated to the station "B" that is the signal which is simply to be relayed to the station "C". Hence, the number of lines to the transmission line must be reserved by taking into account such traffic of signals.
In the case of high-speed digital lines and integrated services digital networks (ISDNs), the transmission is generally made at 32 kbps or 16 kbps per voice grade line using the voice compression technique. In these cases, a voice compression circuit is provided in the transmitting apparatus 2 in a connection channel to the switching system 1, so that the voice compression is carried out prior to transmission to the transmission line. However, the signal which is simply to be relayed by the relay station "B" is also passed through such a voice compression circuit when connecting to the switching system 1 of the relay station "B" although the signal which is to be relayed by the relay station "B" originally does not need to be subjected to the voice compression. On the other hand, in the case where the voice signal has a relatively low bit rate which is less than 16 kbps, the voice signal is subjected to a predetermined voice signal processing other than the voice compression.
In other words, because the switching system 1 processes each voice grade line at 64 kbps, the necessary switching cannot be made if the signals are in the form of the compressed voice signals. Accordingly, even if the compressed voice signals are simply to be relayed at the relay station "B" the compressed voice signals must first be expanded back to the voice signals in order to perform the necessary switching. As a result, there are problems in that an unnecessary voice signal processing circuit must be provided, a signal delay is increased by the provision of the voice signal processing circuit, and the system design becomes complex.